There was one more clue saying that the first word of each of those answers (which had a star before the clue) described the number of its clue. So 4 is even, 6 is perfect, 11 is odd, 25 is square, 33 is real, and 37 is prime.

33 down seems like a bit of a cop-out to me. But I'm not saying I could do better at making a crossword. Crosswords (especially American-style ones) are hard to make; read the information-theoretic argument in MacKay's book for some justification why.

For the non-mathematicians who may have stumbled in (and the mathematicians who don't remember this particular bit of trivia), I feel like I should point out what a perfect number is. A number is perfect if it's equal to the sum of all the numbers it's divisible by. So 6 is divisible by 1, 2, and 3, and 1 + 2 + 3 = 6. 28 is the next perfect number; it's divisible by 1, 2, 4, 7, and 14, and 1 + 2 + 4 + 7 + 14 = 28. But 12 isn't perfect; it's divisible by 1, 2, 3, 4, and 6, and 1 + 2 + 3 + 4 + 6 = 16, which isn't 12. We call 12 "abundant" because 16 (the sum of its divisors) is more than 12. Just under one quarter of integers are abundant, which is entirely irrelevant.